.\" Copyright (c) 1985 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)dhu.4	6.2.1 (2.11BSD) 1997/5/31
.\"
.TH DHU 4 "May 31, 1997"
.UC 2
.SH NAME
dhu \- DHU-11 communications multiplexer
.SH SYNOPSIS
.ft B
.nf
/sys/conf/SYSTEM:
	NDHU	\fIdhu_units\fP	# DHU11

/etc/dtab:
.ta .5i +\w'#Name 'u +\w'Unit# 'u +\w'177777 'u +\w'Vector 'u +\w'Br 'u +\w'xxxxxxx 'u +\w'xxxxxxx 'u
	#Name	Unit#	Addr	Vector	Br	Handler(s)		# Comments
	du	?	160020	310	5	dhurint	dhuxint	# dhu11 terminal mux
.DT

major device number(s):
	raw: 4
minor device encoding:
	bits 0017 specify line on DHU unit
	bits 0060 specify DHU unit
	bit  0100 specify RTS/CTS flow control
	bit  0200 specifies non-blocking open (``CD always on'')
.fi
.ft R
.SH DESCRIPTION
A DHU-11 provides 16 communication lines.
.PP
Each line attached to the DHU-11 communications multiplexer
behaves as described in
.IR tty (4).
Input and output for each line may independently
be set to run at any of 13 speeds (50, 200 and 38400 baud are not available);
see
.IR tty (4)
for the encoding.
.PP
Bit
.I 0200
of the minor device number for DHU lines
may be set to say that a line is not properly
connected, and that the line should be treated as hard-wired with carrier
always present.  Thus creating the special character device node "4, 130" via
.I "mknod /dev/ttyS2 c 4 130"
would cause line ttyS2 to be treated in this way.  Turning on bit 6 (adding
64) to the minor device number via
.I "mknod /dev/ttyS2 c 4 194"
enables RTS/CTS flow control.
.PP
The DHU-11 driver normally uses input silos
and delays receiver interrupts by 20 milliseconds
rather than taking an interrupt on each input character.
.SH FILES
/dev/tty[S-Z][0-9a-f]
.SH "SEE ALSO"
tty(4)
.SH DIAGNOSTICS
\fBdhu(%d,%d) NXM\fR.  No response from UNIBUS on a DMA transfer
within a timeout period.  This has never been observed on a PDP-11 and is
believed to be a carryover from the VAX driver when it was ported.
It is not serious.
.PP
\fBdhu%d %d overruns\fR.  The character input silo overflowed
before it could be serviced.  This message is printed only at line close time
rather than on each overrun error.  Kernel printf's are not interrupt driven
and caused more overruns by blocking interrupts for lengthy periods of time.
.SH NOTES
The driver currently does not make full use of the hardware
capabilities of the DHU-11, for dealing with XON/XOFF flow-control or hard-wired
lines for example.
.PP
The 4 40-way cables are a pain.
